Rotor for rotary electric machine

ABSTRACT

A rotor for a rotary electric machine includes a rotor core including at least one low-rigidity portion rigidity; a plurality of magnets; a rotary shaft; a washer contacting at least one end surface of the rotor core in an axial direction of the rotor core; and a nut. An outer circumferential end of the washer is disposed more radially inward than the magnet holes, the washer has an outer circumferential shape with recesses and projections in which a distance from a rotation center to the outer circumferential end of the washer periodically varies, and a distance from the outer circumferential end of the washer to each low-rigidity portion is greater than a distance from the outer circumferential end of the washer to each magnet hole.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2017-051869 filed onMar. 16, 2017 is incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present specification discloses a rotor for a rotary electricmachine with a washer interposed between a nut screwed onto a rotaryshaft and a rotor core.

2. Description of Background Art

In general, a rotor core is composed by stacking a plurality of magneticsteel plates. It has been conventionally proposed that such a rotor coreis fastened by a nut screwed onto a rotary shaft so as to apply acompression force in the axial direction to the rotor core. Further, ithas also been proposed that a generally annular washer is interposedbetween this nut and the rotor core. By interposing the washertherebetween, a fastening force by the nut is applied to the rotor corevia this washer. Normally, the rotary shaft is inserted through thewasher, and an outer circumferential end of the washer is located moreradially inward than the magnets so as to prevent magneticshort-circuit.

Meanwhile, usually, a rotor core is formed with a plurality of magnetholes extending through the rotor core in the axial direction so as toinsert magnets into these magnet holes. In addition to the magnet holes,the rotor core is also formed with through-holes used for regulatingmagnetic flux flows and forming magnetic resistance, in some cases. Dueto these through-holes, there are low-rigidity portions the rigiditiesof which are locally decreased in the rotor core. For example, in orderto arrange two magnets in a V-shape, two magnet holes in a V-shape areformed in the rotor core. It can be said that a fine gap between thesetwo magnet holes, that is, a portion referred to as a bridge is alow-rigidity portion the rigidity of which is locally decreased.Japanese Patent Application Publication No. 2015-56911 mentions a rotorhaving such bridges.

SUMMARY

When a washer is pushed against an axial end surface of the rotor core,a stress is generated inside the rotor core due to this pushing force(axial force). If a position pushed by the washer is close tolow-rigidity portions, a stress is concentrated onto these low-rigidityportions, so that deterioration or breakage is caused to thelow-rigidity portions. Hence, conventionally, a generally annular washeris configured to have a smaller diameter so as not to set its outercircumferential end too close to the low-rigidity portions.Unfortunately, when the washer is configured to have a smaller diameter,a contact area between the washer and the rotor core becomes smaller,and thus there is such a concern that a sufficient axial force cannot beapplied to the entire rotor core.

To cope with this, the present disclosure provides a rotor for a rotaryelectric machine capable of reducing stress applied to low-rigidityportions, while securing a sufficient axial force.

An example aspect of the present disclosure is a rotor for a rotor for arotary electric machine. The rotor for a rotary electric machineincludes: a rotor core being a generally annular shape having a shafthole at a center, the rotor core having a plurality of magnet holesarranged in a circumferential direction, the rotor core including atleast one low-rigidity portion the rigidity of which is locallydecreased in the rotor core; a plurality of magnets disposed in therespective magnet holes; a rotary shaft fixedly attached to the shafthole of the rotor core; a washer tight contacting with at least one endsurface in an axial direction of the rotor core; and a nut screwed ontothe rotary shaft such that the washer is pushed against the rotor core.An outer circumferential end of the washer is disposed more radiallyinward than the magnet holes. The washer has an outer circumferentialshape with recesses and projections in which a distance from a rotationcenter to the outer circumferential end of the washer periodicallyvaries. A distance from the outer circumferential end of the washer toeach low-rigidity portion is greater than a distance from the outercircumferential end of the washer to each magnet hole.

By configuring the washer to have an outer circumferential shape withrecesses and projections in which the distance from the rotation centerto the outer circumferential end periodically varies, and by setting adistance from the outer circumferential end of the washer to eachlow-rigidity portion to be greater than the distance from the outercircumferential end of the washer to each magnet hole, it is possible toapply an axial force in a wide range of the rotor core, whilesuppressing stress concentration onto the low-rigidity portions. As aresult, it is possible to reduce stress applied to the low-rigidityportions, while securing a sufficient axial force.

The rotor core may include a plurality of pairs of magnet holes in thecircumferential direction, each pair of the magnet holes arranged in aV-shape opening radially outward. Each bridge as a fine gap may beprovided in between each pair of the magnet holes, and the low-rigidityportion may be the bridge.

With such a configuration, also in the rotor including the magnetsdisposed in a V-shaped arrangement, it is possible to reduce stressapplied to the low-rigidity portions, while securing a sufficient axialforce.

The rotor core may include the magnet holes, each magnet hole being longin the circumferential direction, and the low-rigidity portion may bethe gap between the magnet holes adjacent to each other in thecircumferential direction.

With such a configuration, also in the rotor including the magnetsdisposed in an I-shaped arrangement, it is possible to reduce stressapplied to the low-rigidity portions, while securing a sufficient axialforce.

The washer may be disposed such that in the same phase as a phase ofeach low-rigidity portion, a distance from the rotation center to theouter circumferential end becomes minimum.

Through this, the distance from the outer circumferential end of thewasher to each low-rigidity portion can be easily greater, to thus moresecurely suppress stress concentration onto the low-rigidity portions.

The rotor core may have a plurality of sets of through-holes in thecircumferential direction, each set of the through-holes including: apair of the magnet holes arranged circumferentially adjacent to eachother in a V-shape opening radially outward; and an intermediate holebeing disposed between the pair of the magnet holes. Each low-rigidityportion may be a bridge that is the fine gap between each magnet holeand each intermediate hole.

With such a configuration, even in the rotor formed with theintermediate holes, it is possible to reduce stress applied to thelow-rigidity portions, while securing a sufficient axial force.

The washer may be disposed such that in the same phase as a phase of theintermediate hole, the distance from the rotation center to the outercircumferential end becomes minimum.

Through this, the distance from the outer circumferential end of thewasher to each low-rigidity portion can easily be greater, to thus moresecurely suppress stress concentration onto the low-rigidity portions.

The outer circumferential shape of the washer may be a gear-like shapehaving round corners.

The outer circumferential shape of the washer does not have sharpcorners, to thus prevent overconcentration of stress onto a single localposition.

One of an inner circumferential surface of the washer and an outercircumferential surface of the rotary shaft may include a key-projectionprojecting to the other circumferential surface, and the other of theinner circumferential surface of the washer and the outercircumferential surface of the rotary shaft may have a key-grooveaccepting the key-projection.

With such a configuration, it is possible to securely restrict the phaseof the washer relative to the rotor core.

According to the above configuration, it is understood that the washerhas an outer circumferential shape with recesses and projections inwhich the distance from the rotation center to the outer circumferentialend periodically varies, and the distance from the outer circumferentialend of the washer to each low-rigidity portion is set to be greater thanthe distance from the outer circumferential end of the washer to eachmagnet hole. It is possible to apply an axial force in a wide range ofthe rotor core, while suppressing stress concentration onto thelow-rigidity portions. Accordingly, it is possible to reduce stressapplied to the low-rigidity portions, while securing a sufficient axialforce.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like numerals maydenote like elements, and wherein:

FIG. 1 is a schematic longitudinal sectional view of a rotor accordingto an exemplary embodiment;

FIG. 2 is a view of the rotor as viewed from a Y direction;

FIG. 3 is an enlarged view of a part A of FIG. 2;

FIG. 4 is a view showing another configuration of a bridge according toanother exemplary embodiment;

FIG. 5 is a view showing a rotor according to another exemplaryembodiment;

FIG. 6 is an enlarged view of a part B of FIG. 5;

FIG. 7 is a view showing a rotor according to another exemplaryembodiment;

FIG. 8 is a view showing a rotor according to another exemplaryembodiment; and

FIG. 9 is a view showing a conventional rotor.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, a configuration of a rotor 10 for a rotary electric machinewill be described with reference to drawings. FIG. 1 is a schematiclongitudinal sectional view of the rotor 10. FIG. 2 is a view of therotor 10 as viewed in a Y direction. In FIG. 2, an outline of a nut 20is illustrated by a two-dot chain line. FIG. 3 is an enlarged view of apart A of FIG. 2.

This rotor 10 is used for a rotary electric machine, for example, athree-phase synchronous rotary electric machine installed in an electricvehicle or the like, as a drive source. The rotor 10 includes a rotorcore 12, permanent magnets 14 embedded in the rotor core 12, a rotaryshaft 16 fixedly attached to the rotor 10, and a washer 18 and a nut 20applying an axial force to the rotor core 12.

The rotor core 12 is a generally annular shaped body having a shaft holeat a center thereof. The rotor core 12 is formed by stacking a pluralityof electromagnetic steel plates (such as silicon steel plates) in theaxial direction. The rotor core 12 is formed with a plurality of magnetholes 22 in the vicinity of an outer circumferential end of the rotorcore 12, and the plurality of magnet holes 22 are arranged withintervals in the circumferential direction. Each magnet hole 22 extendsthrough the rotor core 12, and a permanent magnet 14 composing amagnetic pole of the rotor 10 is disposed in an inside of each magnethole 22.

In this example, the permanent magnets 14 are disposed in a V-shapearrangement. That is, a single magnetic pole 15 is composed by a pair ofpermanent magnets 14 arranged in a V-shape opening radially outward. Inan example of FIG. 2, the rotor 10 has sixteen permanent magnets 14composing eight magnetic poles 15. Each permanent magnet 14 has a flatand generally rectangular shaped cross section, and is magnetized in itsshort-axial direction (substantially a rotor-radial direction). Of thepermanent magnets 14, permanent magnets 14 composing S-magnetic polesare arranged such that S poles are located radially outward, andpermanent magnets 14 composing N-magnetic poles are arranged such that Npoles are located radially outward.

The magnet holes 22 are also disposed in a V-shape arrangement so as toaccept the permanent magnets 14 disposed in a V-shape arrangement. Thatis, the rotor core 12 is provided with multiple pairs (eight pairs inthe illustrated example) of the magnet holes 22 with equal intervals inthe circumferential direction, each pair of the magnet holes 22 beingarranged in a V-shape opening radially outward. Each magnet hole 22 hasa generally rectangular shape a long axial dimension of which is greaterthan that of each permanent magnet 14. Hence, when each permanent magnet14 is inserted into each corresponding magnet hole 22, voids are formedon both sides in the long-axial direction of this permanent magnet 14.Such a void is called a flux barrier, and functions as a magneticresisting portion so as to regulate magnetic characteristics of therotor core.

Two magnet holes 22 belonging to the same magnetic pole are close toeach other in the circumferential direction, and a bridge 24 that is afine gap portion is formed between the both magnet holes 22. It can besaid that this bridge 24 is a low-rigidity portion rigidity of which islocally decreased in the rotor core 12. To such bridges 24 (low-rigidityportions), stress is likely to be concentrated, so that deterioration orbreakage is likely to be caused thereto. Hence, in order to reducestress concentration onto the bridges 24, the washer 18 is configured tobe in a special shape, and this will be described later.

In this example, each bridge 24 is configured as a part of the rotorcore 12, but the bridge 24 may be composed by using a member differentfrom the stacked steel plates composing the rotor core 12. For example,as shown in FIG. 4, it may be configured that a through-hole in agenerally V-shape continued in a single line is formed, and a bridgemember 25 as indicated by a two-dot line in FIG. 4 may be interposed ina valley portion of this V-shape. In this case, a material of the bridgemember 25 is not limited to a specific one, but it is preferable to usea non-magnetic one for this material.

The rotary shaft 16 is inserted into the shaft hole of the rotor core12, and is fixedly attached thereto. The rotary shaft 16 is rotatablysupported via a not-illustrated bearing, and integrally rotates with therotor core 12. A flange 26 protruding radially outward is formed in themiddle of the rotary shaft 16. The rotor core 12 through which therotary shaft 16 is inserted is pushed against this flange 26.

An outer circumferential surface of the rotary shaft 16, which islocated in the vicinity of the opposite side to the flange 26 with therotor core 12 interposed therebetween, is formed with a male screw. Boldlines in FIG. 1 indicate a position where the male screw is formed. Asdescribed later, the nut 20 is screwed onto this male screw. In thisdrawing, the rotary shaft 16 is illustrated in a hollow cylindricalshape, but the configuration of the rotary shaft 16 is not limited to aspecific one as far as the rotary shaft 16 is concentric to the rotorcore 12, and has an outer circumferential surface having a circularcross section. Hence, the rotary shaft 16 may be as a solid round barshape, or may be formed with a coolant flow passage therein.

The washer 18 and the nut 20, together with the flange 26, restrictmovement of the rotor core 12 in the axial direction, and also applies acompression force in the axial direction to the rotor core 12. Therotary shaft 16 is inserted through the washer 18 from its end portionopposite to the flange 26 so that the washer 18 comes into contact withan axial end surface of the rotor core 12. The nut 20 is mounted ontothe rotary shaft 16 from its end portion opposite to the flange 26 so asto be screwed with the male screw formed on the rotary shaft 16. Bybeing fastened with this nut 20, the rotor core 12 is held between thewasher 18 and the flange 26 so as to restrict the movement in the axialdirection, and also receive a compression force (axial force) in theaxial direction.

Here, in the rotor 10 disclosed in the present specification, for thepurpose of preventing local deterioration of the rotor core 12, or thelike while sufficiently securing the axial force applied to the rotorcore 12, the washer 18 is configured to have a special shape. This willbe described in comparison with the background art. FIG. 9 is an exampleof the rotor 10 of the background art.

When the axial force applied to the rotor core 12 becomes decreased, afixing force for the rotor core 12 becomes naturally decreased. When theaxial force is decreased, gaps between the electromagnetic steel platescomposing the rotor core 12 become greater, so that leakage of thecoolant from the gaps occurs. Hence, it is desired to apply a sufficientaxial force to the rotor core 12.

In order to enhance the axial force applied to the rotor core 12,naturally, a fastening force by the nut 20 may be increased. However, inthe configuration of the background art, the washer 18 is configured tohave an annular shape the diameter of which is substantially the same asthat of the nut 20, so that the washer 18 is in contact with the rotorcore 12 by a radially inward area one third as large as the rotor core12. Hence, if the fastening force by the nut 20 is increased, a greatfastening force (axial force) is applied to only this radially inwardarea one third as large as the rotor core 12; consequently, it isdifficult to apply a sufficient axial force to the entire rotor core 12.

To cope with this, it is natural to consider to increase the area of thewasher 18. However, in order to prevent a magnetic short-circuit andheat radiation of the permanent magnet 14, as well as otherconsiderations, it is preferable that the washer 18 does not cover thepermanent magnet 14. To cope with this, for example, it can beconsidered that the outer diameter of the washer 18 in an annular shapeis enlarged to the vicinity of the inner circumferential ends of themagnet holes 22. With such a configuration, a fastening force by the nut20 can be distributed in a wider area, and thus it is possible to applya sufficient axial force to the entire rotor core 12.

However, if the outer diameter of the annular washer 18 is simplyenlarged, force is applied to the vicinities of the bridges 24 therigidities of which are locally decreased. In this case, a stress causedin the rotor core 12 by applying the fastening force is likely to beconcentrated on the bridges 24 the rigidities of which are decreased.Consequently, other problems, such as deterioration and breakage of thebridges 24, are caused.

To cope with this, in the rotor 10 disclosed in the presentspecification, the washer 18 is configured to have a special shape. Thatis, as shown in FIG. 2 and FIG. 3, the outer circumferential shape ofthe washer 18 of this example is located more radially inward than themagnet holes 22, and a distance from a rotation center O to the outercircumferential end of the washer 18 periodically varies in thecircumferential direction so as to have a shape with recesses andprojections. In addition, as shown in FIG. 3, the washer 18 has such ashape that a distance D1 from the outer circumferential end of thewasher 18 to each bridge 24 is not less than a distance D2 from theouter circumferential end of the washer 18 to each magnet hole 22. Here,each of the distance D1 and the distance D2 means a minimum distance.For example, the “distance D2 from the outer circumferential end of thewasher 18 to each magnet hole 22” means a width of the gap at a positionwhere the width of the gap between the outer circumferential end of thewasher 18 to this magnet hole 22 becomes the smallest. Similarly, the“distance D1 from the outer circumferential end of the washer 18 to eachbridge 24” means a width of the gap at a position where the width of thegap between the outer circumferential end of the washer 18 and thisbridge 24 becomes the smallest.

The shape of the washer 18 is more specifically described as follows.The outer circumferential shape of the washer 18 is a generallyflower-like shape or a generally gear-like shape having round corners.The generally flower-like shape and the generally gear-like shape eachinclude projecting portions 18 a, each in an arc shape projectingradially outward, and recessed portions 18 b, each in an arc shaperecessed radially inward, the projecting portions 18 a and the recessedportions 18 b being arranged alternately with each other in thecircumferential direction. Such a variable period of the recesses andprojections of the outer circumferential shape of the washer 18coincides with an arrangement pitch of the magnetic poles 15, or anarrangement pitch of the bridges 24 as the low-rigidity portions. In thepresent example, there are eight magnetic poles 15 and eight bridges 24,and each arrangement pitch thereof is 360/8=45°; therefore, theprojections and the recesses of the outer circumferential shape of thewasher 18 vary with a period of 45°.

In addition, the washer 18 is disposed such that a center point(hereinafter, referred to as a “recess point 19”) of each recessedportion 18 b, where a distance from the rotation center to the outercircumferential end of the washer 18 becomes minimum, is located in thesame phase as that of each bridge 24. In other words, the washer 18 isdisposed such that each recess point 19 and each bridge 24 are alignedin a straight line in the radial direction.

Hence, the outer circumferential shape of the washer 18 is similar to ashape formed by connecting lines obtained by offsetting innercircumferential ends of the magnet holes 22 disposed in a generallyV-shape arrangement toward a radially inward direction. However, thewasher 18 has an opening angle α1 at a position corresponding to theV-shape that is smaller than an opening angle α2 at this position ofeach magnet hole 22, so that the washer 18 has a V-shape deeper radiallyinward than that of the magnet hole 22. As a result, the distance D1from the outer circumferential end of the washer 18 to the bridge 24 isnot less than a distance D2 from the outer circumferential end of thewasher 18 to the magnet hole 22.

In this manner, since the distance from the outer circumferential end ofthe washer 18 to each bridge 24 becomes greater, it becomes moredifficult to apply force to the vicinity of each bridge 24 rigidity ofwhich is locally decreased, thus suppressing stress concentration ontothe bridge 24. Accordingly, it is possible to effectively suppressdeterioration and damages of the bridges 24. In the meantime, of thewasher 18, each portion the phase of which deviates from the bridge 24extends more radially outward than the rotor 10 of the background art,and thus an axial force can be applied in a wide range of the rotor core12. Accordingly, according to the washer 18 of the present example, itis possible to prevent local deterioration of the rotor core 12, or thelike, while securing a sufficient axial force applied to the rotor core12.

In order to suppress the stress concentration onto the bridges 24, it isnecessary to align the phase of each recess point 19 of the washer 18with the phase of each bridge 24. Hence, in the rotor 10 disclosed inthe present specification, in order to align the phase of the washer 18with the phase of the rotor core 12, the washer 18 is key-engaged withthe rotary shaft 16. Specifically, an inner circumferential surface ofthe washer 18 is provided with key-projections 30 projecting radiallyinward, and the outer circumferential surface of the rotary shaft 16 isprovided with key-grooves 32 accepting the key-projections 30. Thewasher 18 is assembled to the rotary shaft 16 such that thekey-projections 30 are fitted into the key-grooves 32, to therebyrestrict the phase of the washer 18 relative to the rotor core 12 in apreferable manner.

In the present example, the washer 18 is provided with thekey-projections 30, and the rotary shaft 16 is provided with thekey-grooves 32, but this combination may be inverted to each other. Thismeans that the rotary shaft 16 may be provided with the key-projections,and the washer 18 may be provided with the key-grooves. The number ofthe key-grooves 32 and the number of the key-projections 30 mayappropriately be changed.

Next, another example of the rotor 10 will be described. FIG. 5 is aview showing one example of a rotor 10 according to another exemplaryembodiment. FIG. 6 is an enlarged view of a part B of FIG. 5. In thisrotor 10, each single magnetic pole 15 is composed by three permanentmagnets 14 a, 14 b arranged in an inverted triangle shape (subscriptsare omitted when the permanent magnets 14 a, 14 b of two types are notdistinguished from each other. This is the same in the magnet holesdescribed later). That is, each single magnetic pole 15 is composed by apair of the permanent magnets 14 a arranged in a V-shape openingradially outward and one permanent magnet 14 b disposed betweenrespective outer circumferential ends of the pair of the permanentmagnets 14 a.

The rotor core 12 is formed with the magnet holes 22 for accepting thepermanent magnets 14. Each pair of the magnet holes 22 a are arranged ina V-shape opening radially outward. Note that these two magnet holes 22a are not close to each other in the circumferential direction, butthere is an intermediate hole 28 between respective innercircumferential ends of the two magnet holes 22 a. Each intermediatehole 28 is a through-hole extending through the rotor core 12 in theaxial direction. This intermediate hole 28 functions as a magneticresisting portion, and by providing the intermediate hole 28, magneticflux flows can be regulated. Here, as apparent from FIG. 6, the bridge24 as a fine gap portion rigidity of which is locally decreased isformed between each intermediate hole 28 and each magnet hole 22 a.

In addition, a magnet hole 22 b extending in the circumferentialdirection is formed between respective outer circumferential ends ofevery two magnet holes 22 a. A pair of void holes 29 are formed on bothcircumferential sides of each magnet hole 22 b. As similar to theintermediate hole 28, each void hole 29 is a through-hole extendingthrough the rotor core 12 in the axial direction, and functions as amagnetic resisting portion. This void hole 29 is also provided so as toregulate the magnetic flux flows.

In examples shown in FIG. 5 and FIG. 6, the washer 18 is disposed on theaxial end surface of the rotor core 12, and the washer 18 is pushedagainst the rotor core 12 with a fastening force by the nut 20 (notillustrated in FIG. 5 and FIG. 6). As similar to the example shown inFIG. 2, the outer circumferential shape of the washer 18 of this exampleis located more radially inward than the magnet holes 22, and thedistance thereof from the rotation center O to the outer circumferentialend periodically varies in the circumferential direction so as to have ashape with recesses and projections. In addition, as shown in FIG. 6,the washer 18 has such a shape that a distance D1 from the outercircumferential end of the washer 18 to each bridge 24 is not less thana distance D2 from the outer circumferential end of the washer 18 toeach magnet hole 22.

To be more specific, the outer circumferential shape of the washer 18 isa generally flower-like shape or a generally gear-like shape havinground corners. The generally flower-like shape and the generallygear-like shape each include the projecting portions 18 a, each in anarc shape projecting radially outward, and the recessed portions 18 b,each in an arc shape recessed radially inward, the projecting portions18 a and the recessed portions 18 b being arranged alternately with eachother in the circumferential direction. The variable period of therecesses and projections of the outer circumferential shape of thewasher 18 coincides with the arrangement pitch of the magnetic poles 15.

In addition, the washer 18 is disposed such that each recess point 19,where a distance from the rotation center to the outer circumferentialend of the washer 18 becomes minimum, is located in the same phase asthat of the circumferential center of the intermediate hole 28. In otherwords, the washer 18 is disposed such that each recess point 19 and thecircumferential center of each intermediate hole 28 are aligned in astraight line in the radial direction. With such a configuration, it ispossible to reduce the stress concentration onto the bridges 24, and itis also possible to apply an axial force in a wide range of the rotorcore 12.

Note that each recessed portion 18 b is formed in a generally arc shaperecessed radially inward in the examples of FIG. 5 and FIG. 6, but thisrecessed portion 18 b may be a straight shape extending in the generallycircumferential direction, as shown in FIG. 7. With such aconfiguration, compared with the cases of FIG. 5 and the FIG. 6, thedistance D1 from the outer circumferential end of the washer 18 to thebridge 24 can be greater, to thus suppress more stress concentrationonto each bridge 24.

Next, another exemplary embodiment of the rotor 10 will be described.FIG. 8 is a view showing another exemplary rotor 10. In this rotor 10,each single magnetic pole 15 is formed by a single permanent magnet 14that is long in the generally circumferential direction. In the exampleof FIG. 8, the rotor 10 includes sixteen magnetic poles 15 and sixteenpermanent magnets 14.

The rotor core 12 is formed with the magnet holes 22 for accepting thesepermanent magnets 14. Each magnet hole 22 is a flat rectangular shapethat is long in the generally circumferential direction, and a longaxial dimension is greater than a long axial dimension of the permanentmagnet 14. A gap portion 36 having a thin width is formed between eachtwo magnet holes 22 adjacent to each other in the circumferentialdirection. It can be said that this gap portion 36 is a low-rigidityportion rigidity of which is locally decreased, as with the abovedescribed bridge 24.

As with the washer 18 shown in FIG. 2, the washer 18 in tight contactwith this rotor core 12 has an outer circumferential shape located moreradially inward than the magnet holes 22, and having recesses andprojections in which the distance from the rotation center O to theouter circumferential end periodically varies in the circumferentialdirection. In addition, as shown in FIG. 8, the washer 18 has such ashape that the distance D1 from the outer circumferential end of thewasher 18 to each gap portion 36 is not less than the distance D2 fromthe outer circumferential end of the washer 18 to each magnet hole 22.

To be more specific, the outer circumferential shape of the washer 18 isa generally flower-like shape or a generally star-like shape havinground corners. The generally flower-like shape and the generallystar-like shape each include projecting portions 18 a, each in an arcshape projecting radially outward, and recessed portions 18 b, each inan arc shape recessed radially inward, the projecting portions 18 a andthe recessed portions 18 b being arranged alternately with each other inthe circumferential direction. The variable period of the recesses andprojections of the outer circumferential shape of the washer 18coincides with the arrangement pitch of the magnetic poles 15.

In addition, the washer 18 is disposed such that the circumferentialcenter point of each recessed portion 18 b (hereinafter, referred to asa “recess point 19”), where the distance from the rotation center O tothe outer circumferential end of the washer 18 becomes minimum, islocated in the same phase as that of the gap portion 36. In other words,the washer 18 is disposed such that each recess point 19 and the gapportion 36 are aligned in a straight line in the radial direction. Withsuch a configuration, it is possible to reduce the stress concentrationonto the gap portions 36, and it is also possible to apply an axialforce in a wide range of the rotor core 12.

As apparent from the above description, according to the rotor 10disclosed in the present specification, it is possible to reduce stressapplied onto the low-rigidity portions. The configurations disclosed inthe present specification are exemplary embodiments, and as far as thedistance from the outer circumferential end of the washer to eachlow-rigidity portion is not less than the distance from the outercircumferential end of the washer to each magnet hole, the otherconfigurations may appropriately be changed. For example, in the abovedescription, the washer 18 is formed in a generally gear-like shapecorners of which are all round, but may be formed in a generallygear-like shape having sharp corners. However, in order to avoidoverconcentration of stress onto a single local position, it ispreferable to avoid sharp corners for the outer circumferential shape ofthe washer 18, as much as possible.

What is claimed is:
 1. A rotor for a rotary electric machine comprising:a rotor core having a generally annular shape with a shaft hole at acenter, the rotor core including a plurality of magnet holes arranged ina circumferential direction, the rotor core including at least onelow-rigidity portion; a plurality of magnets disposed in the respectivemagnet holes; a rotary shaft fixedly attached to the shaft hole of therotor core; a washer flush with at least one end surface of the rotorcore in an axial direction of the rotor core; and a nut screwed onto therotary shaft such that the washer is pushed against the rotor core,wherein an outer circumferential end of the washer is disposed moreradially inward than the magnet holes, the washer has an outercircumferential shape with recesses and projections in which a distancefrom a rotation center to the outer circumferential end of the washerperiodically varies, and a distance from the outer circumferential endof the washer to each low-rigidity portion is greater than a distancefrom the outer circumferential end of the washer to each magnet hole. 2.The rotor for the rotary electric machine according to claim 1, whereinthe rotor core includes a plurality of pairs of magnet holes in thecircumferential direction, each pair of the magnet holes arranged in aV-shape opening radially outward, each bridge is a gap provided betweeneach pair of the magnet holes, and the bridge is the low-rigidityportion.
 3. The rotor for the rotary electric machine according to claim2, wherein the washer is disposed such that when each recess is in thesame phase as a phase of each low-rigidity portion, a distance from therotation center to the outer circumferential end becomes smallest. 4.The rotor for the rotary electric machine according to claim 1, whereinthe rotor core includes the magnet holes, each magnet hole extending inthe circumferential direction, and the low-rigidity portion is the gapbetween the magnet holes adjacent to each other in the circumferentialdirection.
 5. The rotor for the rotary electric machine according toclaim 4, wherein the washer is disposed such that when each recess is inthe same phase as a phase of each low-rigidity portion, a distance fromthe rotation center to the outer circumferential end becomes smallest.6. The rotor for the rotary electric machine according to claim 1,wherein the rotor core has a plurality of sets of through-holes in thecircumferential direction, each set of the through-holes including: apair of the magnet holes arranged circumferentially adjacent to eachother in a V-shape opening radially outward; and an intermediate holebeing disposed between the pair of the magnet holes, and eachlow-rigidity portion is a bridge that is a gap between each magnet holeand each intermediate hole.
 7. The rotor for the rotary electric machineaccording to claim 6, wherein the washer is disposed such that when eachrecess is in the same phase as a phase of the intermediate hole, thedistance from the rotation center to the outer circumferential endbecomes smallest.
 8. The rotor for the rotary electric machine accordingto claim 1, wherein the outer circumferential shape of the washer is agear-like shape having round corners.
 9. The rotor for the rotaryelectric machine according to claim 1, wherein one of an innercircumferential surface of the washer and an outer circumferentialsurface of the rotary shaft includes key-projections projecting towardthe other of the inner circumferential surface of the washer and theouter circumferential surface of the rotary shaft, and the other of theinner circumferential surface of the washer and the outercircumferential surface of the rotary shaft has key-grooves acceptingthe key-projections.
 10. The rotor for the rotary electric machineaccording to claim 1, wherein the at least one low-rigidity portion isan area of the rotor that has a rigidity less than the rigidity ofanother portion of the rotor.
 11. A rotor for a rotary electric machinecomprising: a rotor core having a generally annular shape including ashaft hole along a central axis, a plurality of magnet holes arrangedcircumferentially around the rotor core, and at least one low-rigidityportion; a plurality of magnets disposed in the respective magnet holes;a rotary shaft disposed within the shaft hole; a washer flush with atleast one end surface of the rotor core in an axial direction of therotor core; and a nut provided on the rotary shaft such that the washeris continuously urged against the rotor core, wherein an outercircumference of the washer includes recesses and projections such thata distance from a rotation center to the outer circumferential end ofthe washer varies, and a distance from the outer circumferential end ofthe washer to each low-rigidity portion is greater than a distance fromthe outer circumferential end of the washer to each magnet hole.
 12. Therotor for the rotary electric machine according to claim 11, wherein theat least one low-rigidity portion is an area of the rotor that has arigidity less than the rigidity of another portion of the rotor.